Tannins are a group of naturally occurring compounds that exist in various plants, for example, Rhus chinensis, Rhus javanica, Rhus semialata, Rhus coriaria, Rhus potaninii, Rhus punjabensis var. sinica (Diels) Rehder & E. H. Wilson, Camellia sinensis, Berry, Bixa orellana, Vitis vinifera, Punica granatum, Quercus infectoria, Quercus cerris, Acacia mearnsii, Pseudotsuga menziesii, Caesalpinia spinosa, Fagus hayata Palib. ex Hayata, or Machilus thunbergii Sieb. & Zucc. etc. There are three major classes of tannins, including hydrolysable tannins (also known as tannic acids), condensed tannins, and phlorotannins, which contain gallic acid, flavone, and phloroglucinol, respectively, as the base unit. Tannins are widely used as a type of industrial particleboard adhesive and for production of anti-corrosive primer or resins. It was also suggested that tannins may have various effects on human health.
D-amino acid oxidase (DAAO) is a peroxisomal enzyme that oxidizes D-amino acids to the corresponding imino acids. It has been reported that DAAO is involved in the metabolism of brain D-amino acids, including D-serine, and the regulation of the glutamatergic neurotransmission. As such, DAAO is a target for treating central nervous system (CNS) disorders that are associated with D-serine and/or glutamatergic neurotransmission.